In soldering electronic components, circuits, equipment and the like, various kinds of fluxes are used together with soldering material so as to improve the efficiency of the soldering operation, to secure the soldered connections and to improve the long-term reliability of the connections. Conventionally, there are three broad categories of flux: (1) natural rosin, (2) activated rosin having a halogenated compound, incorporated with natural rosin, and (3) rosin free flux, generally referred to as water soluble flux.
Natural rosin flux is a stable flux but results in a solid rosin flux residue which, if not completely removed, results in serious contact resistance problems in electronic relays, connectors, gold fingers and printed wire board (PWB) circuitry. It presents few problems with respect to corrosiveness.
Activated rosin flux has a stability similar to natural rosin flux and causes little corrosion at room temperature. Fully activated fluxes have a strong fluxing action at soldering temperatures due to activators, such as an amine hydrochloride, which are typically present in high concentrations such as 1 to 10 weight percent of the resultant flux. However, the fully activated rosin flux has disadvantages in that at soldering temperatures a corrosive gas is produced. Moreover, the residues of the activated rosin combine with moisture to produce corrosive acid. Presently available fluxes containing organic amine hydrohalides, in the form of neutral salts such as glutamic acid hydrochloride, either form corrosive metal halides at elevated temperature or the residues thereof combine with moisture at room temperature to form a corrosive acid and thus are used with possible deleterious effect for electrical soldering applications.
There are also mildly activated rosin fluxes such as those taught in U.S. Pat. No. 4,168,996. While these are not corrosive, they still suffer from the same disadvantage of rosin flux in that the flux residue is difficult to remove.
Rosin free fluxes are very useful in removing oxides from the metal surface to be soldered. They have the distinct advantage of being easily removed from the device by simple washing with a suitable solvent or detergent. However, there are disadvantages to their use because they contain either a free mineral acid, such as hydrochloric acid, or an inorganic salt, which hydrolyzes in water to give an acid reaction. Therefore, they are apt to destroy metallic material or to leave residues which corrode the soldered parts after soldering, resulting in decreased reliability of the soldered parts with respect to electrical and mechanical properties.
Furthermore, some rosin free fluxes comprise polyethylene glycol and many of its derivatives as flux vehicles. It has been found that such fluxes interact with the plastic surfaces of electronic devices, e.g., in printed wiring boards, to modify them so that they become more conductive, thereby increasing still further the chances of device malfunctioning. The change in substrate surface quality with regard to conduction is measurable as a decrease in insulation resistance.
Therefore, there is a need for a rosin free soldering flux composition that is noncorrosive although mildly activated and which does not adversely react with the substrate of a device to be soldered so as to degrade its insulation resistance.